PALEOZOIC DYKE SWARM VOLCANISM IN THE NORTH ARMORICAN DOMAIN

Inside the North Armorican domain, many doleritic dykes cut across the Cadomian basement and its Palaeozoic cover. In Tregor, these dykes ar e roughly E-W and are of two generations: one is Ordovician which is w idespread, the other Silurian is more restricted. In the St-Male area, the dykes run N-S and are Lower Carboniferous. This shows that volcan ism is continuous during the Palaeozoic period in the North-Armorican area. These basalts are typically continental tholeiites. Tregor doler ites show crustal features with trace element ratios and isotopic sign atures [Th/Ta = 3.5; (Sr-87/Sr-86)i = 0.707] whereas St-Male dolerites present more mantellic features [Th/Ta = 1.1; (Sr-87/Sr-86)i=0.703]. Compared with other volcanites known in more southern basins (Central Armorican domain), two petrogenetic and geodynamic models are proposed for Tregor and St-Male areas: (1) Tregor dolerites have a mantle sour ce whose signature is masked by an important crustal contamination and represent the extensional post-Cadomian tectonic phase known in this region; (2) St-Malo dolerites are the result of the mixing of magma ge nerated by partial melting of a heterogeneous mantle and their occurre nce corresponds with the extensional pull-apart tectonic phase known i n the Central Armorican domain.